With increasing demands on computational power and data transmission bandwidth, electronic devices and microstructures incorporating such devices are becoming increasingly complex necessitating a greater degree of mechanical and electrical interconnection among components. In response, three-dimensional microstructures provide a variety of advantages in accommodating the need for increased device performance. By way of example, three-dimensional microstructures and methods for their manufacture are illustrated at least at U.S. Pat. Nos. 7,948,335, 7,405,638, 7,148,772, 7,012,489, 7,649,432, 7,656,256, 7,755,174, 7,898,356, 8,031,037 and/or U.S. Application Pub. Nos. 2010/0109819, 2011/0210807, 2010/0296252, 2011/0273241, 2011/0123783, 2011/0181376 and/or 2011/0181377, each of which is hereby incorporated by reference in their entirety.
A typical approach for electrically and/or mechanically interconnecting both planar and three-dimensional microstructures is soldering. However, it may be difficult to stop solder from wicking up the length of a metal component, especially in view of the complex surface morphologies which may be encountered in three-dimensional microstructures, and particularly when such structures are made of or coated with metals such as gold, silver, copper or similar metals which are capable of promoting solder flow. For instance, the presence of a multitude of discrete components, mounting surfaces, interconnected chips, and so on presents a variety of surface height changes and void spaces prone to wicking molten solder along the surfaces of such components.
While the phenomena of adhesion of a desired solder to base metal may be called “wetting” and lack of it as “non-wetting”, for the purposes of the present application the term “wicking” is defined to connote the flow (intended or unintended) of solder along the surface of parts, even though the physics of the flow is not one of traditional fluidic “wicking” in the sense as it occurs when a fabric contacts water. Wicking therefore in the context of the present application is the wetting of the solder, and to stop the wicking in the present application refers to stopping of wetting and flow of the solder past the intended boundaries. A clean thin layer of gold on platinum, intended for solder reflow, may for example, continue to wet the surface particularly in non-oxidizing conditions for a great distance until the solder thickness or composition due to interdiffusion becomes unacceptable for its intended purpose.
The unintended flow of solder throughout such microstructures may cause decreased performance, uncontrolled bond lines, shorting, solder embrittlement, and other problems. In traditional planar structures such as circuit boards control of the solder flow can be performed with a patterned solder mask. Often such materials are either selectively applied or patterned, e.g., photo-patterned, or they may be micro-sprayed. Whereas a “solder mask”, such as a patterned planar dielectric coating, may be used to stop and/or control solder flow, in an open three-dimensional structure applying such a material may be relatively difficult to achieve for multiple reasons. First, the interconnects and/or electrical junctions where devices are to be attached may be on a layer other than the surface layer, precluding the use of dry film. Second, a complex three-dimensional structure may be hard to coat and/or pattern lithographically on more than one layer. Third, it may be desirable to ensure substantially complete removal of any existing solder mask materials as they may degrade performance such as RF performance, because they may not be applied with sufficient accuracy and/or quantity for many applications (e.g., microwave devices) onto such three-dimensional structures. These problems are aggravated when the desired pad dimensions for a solder or conductive adhesive continue to shrink from squares of hundreds of microns on a side to squares of tens of microns, as currently is the case for some microwave and mm-wave devices and circuits such as MMICs.
In addition, three-dimensional microstructures may include coatings of excellent conductors and/or noble metals, such as gold, which may aggravate a problem of solder flowing along a conductor in an uncontrolled manner. Further, solder thickness and even conductive adhesive thickness, as well as volume, in a particular location often need to be controlled as the these parameters can determine mechanical properties such as strength and resistance to fatigue. Maintaining the solder's reflow over a controlled location during attach can provide compositional control of the metals in the solder system as noble metals, diffusion barriers, and base metals tend to dissolve to varying degrees and therefore impact lifetime and other properties of the electro-mechanical junctions at the points of attach. Still, solder attach may be an important technique for high strength and reliable device attachment. Previous approaches for three-dimensional microstructures have failed to disclose how to maintain adhesion of such coatings particularly when the CTE match of the wettable metals and the non-wettable layers or “wick stop” materials may be highly mismatched. Thus, there remains a need to control flow, wetting area, and/or spread of solder material for three-dimensional micro-electric structures including, for example, those incorporated herein by reference above.